Static vs Kinetic Friction — What's the Difference?
Static Friction
The friction force that prevents motion between surfaces at rest relative to each other. It automatically matches the applied push — up to a maximum value — which is why a heavy box stays put until you push hard enough.
Unit: newton (N)
Example: Push a 40 kg crate gently and static friction pushes back equally — the crate doesn't move. Push harder than μ_s·N and it finally breaks free.
Kinetic Friction
The friction force opposing motion when surfaces slide past each other. Unlike static friction it has a nearly fixed value, independent of speed over a wide range.
Unit: newton (N)
Example: Once the crate is sliding, kinetic friction is roughly constant — you need less force to keep it moving than you needed to start it.
Key differences
| Aspect | Static Friction | Kinetic Friction |
|---|---|---|
| When it acts | Surfaces at rest relative to each other | Surfaces sliding past each other |
| Magnitude | Self-adjusting: equals the applied force up to a maximum | Approximately constant for a given pair of surfaces |
| Coefficient | μ_s (static coefficient) | μ_k (kinetic coefficient) — usually smaller than μ_s |
| Which is larger? | Maximum static friction is typically greater than kinetic | Kinetic is typically the smaller of the two |
| Energy | Does no work (no displacement) | Dissipates mechanical energy as heat |
Students plug μ_s into f = μN for a sliding object. Once it slides, static friction is gone — use μ_k. The familiar 'harder to start than to keep moving' is exactly the statement μ_s > μ_k.